1. Field of the Invention
This invention, in its preferred form, relates to apparatus for effecting a plurality of precision welds utilizing a laser beam while effecting movement of the work piece to be welded through a sequence of controlled movements whereby the work piece is accurately positioned with respect to the laser beam, and each of the plurality of welds is carried out with a precisely controlled quantity of energy. More particularly, this invention relates to apparatus for welding the elements, i.e. grid spacers, of a nuclear fuel assembly made of a volatile metallic material such as the zirconium alloy known as Zircaloy.
2. Description of the Prior Art
The precision laser welding apparatus of this invention relates generally to the manufacture of nuclear fuel bundle assemblies 10 as shown in FIG. 1 of the drawings. As shown, the nuclear fuel bundle assembly 10 is a self-contained unit comprised of a top nozzle assembly 12 and a bottom nozzle assemble 14, between which is disposed a matrix of nuclear fuel rods 18 arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids 16. Though not shown in FIG. 1, control rods are included at selected positions within the array of nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids 16 provide a skeletal frame to support the fuel rods 18 and the control rods. The nuclear fuel bundle assemblies 10 are loaded into predetermined locations within a nuclear reactor and, therefore, the orientation of the fuel rods 18 with respect to each other is rigorously controlled.
The precision laser welding apparatus of this invention is, in one illustrative embodiment thereof, related to the manufacture of fuel rod grids 16 as shown in FIGS. 2A to 2E. The fuel rod grid 16 is of an approximately square configuration, whose periphery is formed by four outer grid straps 22. Each end of an outer grid strap 22 is welded by a corner seam weld 30 to the end of a perpendicularly disposed outer grid strap. A plurality of inner grid straps 20 is disposed in rows and columns perpendicular to each other, whereby a plurality of cells are formed to receive the control rods and the nuclear fuel rods 18. The inner grid straps 20 disposed along the rows and columns have complementary slots therein at each of the points 24 of intersection for receiving a perpendicularly disposed inner grid strap 20. An intersect weld 32 is formed at each of the points 24 of intersection, whereby a rigid egg crate structure is formed. Further, each of the inner grids straps 20 includes at each end a pair of tabs 26 of a size and configuration to be tightly received in either a top or bottom row of slots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slot and tab weld 34 is effected along the top and bottom rows formed by the slots 28 within the outer grid straps 22. Further, a plurality of guide sleeves 36 is disposed on the sleeve side surface of the fuel rod grid 16 to receive and guide the control rods disposed therein. A series of notch seam welds 40 securely attaches the guide sleeves 36 to corresponding notches 38 formed within the inner grid straps 20. The precision laser welding apparatus of this invention is particularly adapted to perform a series of controlled welding operations whereby each of the welds 30, 32, 34 and 40 is carried out. The precision laser welding apparatus of this invention not only controls the various parameters of generating the laser in terms of the pulse width, the pulse height of each laser pulse, and the number of pulses to be applied to each weld, but also controls the sequential positioning of the fuel rod grids 16 with respect to the laser beam. It is understood that after each such weld, the fuel rod grid 16 is repositioned and/or the focal point of the laser beam changed to effect the particular type of weld desired.
Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44 is disposed longitudinally of the inner grid straps 20 in a parallel relationship to each other. A pair of spacing fingers 46 is disposed on either side of a corresponding resilient finger 44 and serves along with the resilient finger 44 to provide a resilient grip of the nuclear fuel rods 18 that are disposed within the cell formed by the intersecting inner grid straps 20. A resilient finger 44a is disposed to the right as seen in FIG. 2C in an opposing relationship to the spacing finger 46a, whereby a nuclear fuel rod 18 is resiliently held therebetween.
The manner of assembling the inner grid straps 20 to each other as well as to the outer grip straps 22 is shown in FIG. 2D. Each of the inner grid straps 20 includes a plurality of complementary slots 52. An upper grid strap 20a has a downwardly projecting slot 52a, whereas a lower grid strap 20b has a plurality of upwardly oriented slots 52b of a configuration and size to be received within a corresponding slot 52a of the inner grid strap 20a. At each end of the inner grid strap 20, there is disposed a pair of the tabs 26 to be disposed within corresponding slots 28 of an outer grid strap 22.
As will be explained in detail later, the inner grid straps 20 are welded to each other by the intersect welds 32 as formed of projection tabs 48 and tab portions 50a and 50b. More specifically, a projection tab 48 is disposed between a corresponding set of tab portions 50a and 50b when the inner grid straps 20a and 20b are assembled together. Upon the application of a laser beam to the tab 48 and tab portions 50a and 50b, an intersect weld 32 is formed that is rigidly strong and free of contamination in accordance with the teachings of this invention. Further, each end of an outer grid strap 22 has a corner tab 54. As shown in FIG. 2D, the outer grid straps 22c and 22b have respectively corner tabs 54b and 54c that overlap each other and are seam welded together to form the corner seam weld 30.
The vanes 42 project, as seen in FIGS. 2C and 2E, from a vane side of the fuel rod grid 16 to enhance the turbulence of the water passing over the nuclear fuel rods 18. Further, as illustrated particularly in FIG. 2C, the guide sleeves 36 are aligned with cells formed by the inner grid straps 20 that are free of either a resilient finger 44 or spacing finger 46, to thereby permit the free movement of the control rod through the cell and through the guide sleeve 36.
U.S. Pat. No. 3,966,550 of Foulds et al., and U.S. Pat. No. 3,791,466 of Patterson et al., assigned to the assignee of this invention, disclose similarly configured fuel rod grids of the prior art. Each of these patents discloses a fuel rod grid wherein the inner and outer grid straps are made of a suitable metallic alloy such as Inconel, and the above identified interconnections are effected by furnace brazing. However, the zirconium alloy Zircaloy is known to have the desirable characteristic of a low neutron absorption cross section which allows for more efficient use of the nuclear fuel in the utility operation and therefore allows for a longer elapsed time between refueling by the replacement of the nuclear fuel bundle assemblies. In particular, fuel rod grids made of Zircaloy have a lower absorption rate of the neutrons generated by the fuel rods than that absorption rate of straps made with Inconel. The making of the grid straps of Zircaloy requires at least several changes in the assembly of the fuel rod grids. First, it is necessary to make the slots, whereby the inner grid straps may intersect with each other, of looser tolerances in that grid straps made of Zircaloy do not permit a force fitting thereof, i.e. to be hammered into position, but rather require controlled fit-up to allow "push-fits" of the intersecting grid straps. In addition, Zircaloy grid straps may not be brazed in that heating Zircaloy to a temperature sufficient to melt the brazing alloy would anneal the Zircaloy, resulting in a loss of mechanical strength.
Prior to the selection of a particular method of welding, several different methods of welding volatile materials such as Zircaloy were investigated including continuous welding with a CO.sub.2 laser, pulsed emission of a Nd:YAG laser, gas tungsten arc welding and electron beam welding. A pulsed electron beam is capable of power densities of up to 10.sup.9 watts/square centimeter with pulse widths in the micro-second and low milli-second range. However, welding with an electron beam is typically carried out in a vacuum environment which is relatively expensive to build and requires a relatively long time to establish the desired degree of vacuum therein, thus slowing down the manufacture of the fuel rod grids. Further, it is necessary to obtain relative movement of the work piece, e.g. the fuel rod grids, in three dimensions with respect to the electron beam which would require a very complex grid positioning system. The use of a continuous electron beam provides relatively low levels of power (in the order of 200 watts) requiring relatively long welding times and providing very shallow weld penetrations. The use of a gas tungsten arc was also considered and proved to be unacceptable for providing a sequence of welds in that after a given number, of welds, e.g. 25, the arc electrodes require sharpening to provide the desired fine arc to produce numerous well defined welds and to avoid damaging adjacent grid straps or vanes of the fuel rod grids. Two types of lasers are commonly used for welding applications: (1) the solid state Nd:YAG laser, which uses a crystal rod of neodynium doped yttrium-aluminum-garnet and (2) the CO.sub.2 laser, which uses a mixture of CO.sup.2 --N.sub.2 --He as the lasing medium. An inherent advantage of the Nd:YAG laser is that its emission is in the order of 1.06 micron wave lengths, where glass is transparent to its laser emission. This characteristic permits the use of a coaxial microscope which uses the same optic elements for both optical viewing and laser focusing. Further, a pulsed Nd:YAG laser is capable of 400 watts of average power, of a pulse frequency of up to 200 pulses per second and of a peak power in excess of 8000 watts for up to 7 milli-seconds. Such high peak power permits the Nd:YAG laser to produce welds of relatively deep penetration, thus insuring the structural security of welded straps of the nuclear fuel rod grids. Such lasers may be operated from a "cold start" with its shutter remaining open, whereby the weld time is determined by the length of time the power is applied to its flash lamps. Such a method of welding is not particularly applicable to a series of relatively rapid welds due to the laser rod warm-up time for each weld in the order of 0.8 seconds. Further, optical path length changes occur until a condition of thermal equilibrium is attained within the laser rod. A second method of operation of the Nd:YAG laser permits the continuous pulse operation of the laser while using its shutter to "pick off" a fixed number of pulses, thus eliminating the effects of laser warm-up and ensuring a uniformity of welds even though a great number of such welds are being effected.
The machining and, in particular, the laser drilling and welding of Zircaloy is described in articles entitled "Pressurization of Nuclear Fuel Rods Using Laser Welding", by Peter P. King and "External Attachment of Titanium Sheathed Thermocouples to Zirconium Nuclear Fuel Rods for the Loss-of-Fluid Test (LOFT) Reactor", both appearing in the proceedings of the Society of Photo-Optical Instrument Engineering, Volume 247, ADVANCES IN LASER ENGINEERING AND APPLICATIONS, (1980). Both of these articles particularly relate to the manufacture of nuclear fuel rods such as those rods 18 shown in FIG. 1. In the article entitled "Pressurization of Nuclear Fuel Rods Using Laser Welding", by Peter P. King, various possible welding techniques other than laser welding are described. In particular, resistance butt welding was attempted but found difficult to control and reproduce when welding thin walled cladding. In turn, high pressure gas tungsten arc welding experienced arc initiation and control difficulties at relatively high pressure. In particular, the nuclear fuel rods are described as being loaded with fuel pellets and sealed by gas tungsten arc welding in high purity helium; thereafter, the fuel rods are introduced into a laser pressurization chamber through a gland seal. The upper end cap of each fuel rod is drilled by a sharply focused laser beam, while the chamber is pressurized with high purity helium. After the laser drilling operation, the helium rushes through the drilled opening and into the rod; thereafter, the drilled hole is sealed by defocusing the laser beam. In addition to providing the desired pressurized gas within the rods, the use of helium within the welding chamber provides a suitable inert gas that will not rapidly oxidize (burn) or contaminate the Zircaloy. Further, a totally automatic fuel rod laser pressurization system is described wherein a tape control system of mini-computer is used to advance the fuel rod into the laser pressurization chamber to its laser welding postion at the focal point of the laser beam, to lock the gland seal, to control the chamber evacuation and introduction of the inert gas helium, and to control the pulse laser operation to effect first the desired hole drilling and thereafter, the hole sealing.
A similar teaching of the laser drilling and sealing of a nuclear fuel rod is described in U.S. Pat. No. 3,774,010 of Heer et al. This patent discloses that the nuclear fuel rod is brought to a single position where it is first drilled and then the drilled hole is sealed. Thus, it is evident that it is necessary to reposition such a work piece or to control a series of lasing operations as would be required to effect the intersect welds 32, the corner seam welds 30, the slot and tab welds 34, and the seam welds 40 of the fuel rod grid 16 as shown in FIG. 2A. Consideration of the number and type of welds required to manufacture the fuel rod grids 16 indicates that it is necessary to move the grid 16 along X and Y axes in a series of steps to effect the intersect welds, whereas it would be necessary to rotate the work piece in the form of the grid 16 from the plane formed by the X and Y axes in order that the notch seam welds 40 and the slot and tab welds 34 may be carried out.
The prior art has recognized the problem of fretting corrosion, wherein the surfaces of the fuel rod grids 16 and the fuel rods 18 rub against each other increasing the likelihood of weld contamination and eventual mechanical failure of the fuel rod grids 16. Fuel bundle assemblies 10 including the fuel rods 18 and grids 16 are designed to be disposed within the hostile atmosphere of a boiling water reactor (BWR) or pressurized water reactor (PWR), wherein the coolant, typically in the form of water, is super heated to temperatures in the order of 600.degree. F., i.e. the boiling point of the water coolant is raised by applying extremely high pressures thereto. Under such conditions, any contamination, and in particular, fretting corrosion is enhanced. A publication entitled "Special Features of External Corrosion of Fuel Cladding in Boiling Water Reactors", by Liv Lunde, appearing in NUCLEAR ENGINEERING AND DESIGN, (1975), describes the various mechanisms responsible for fretting corrosion. First, metallic particles are produced by grinding or by formation of welds at the points of contact between the grid 16 and its fuel rod 18. These metal particles subsequently oxidizes to form an abrasive powder to increase the abrasive action. Finally, the metal beneath the protective oxide layer oxidizes due to the continuous removal of the metallic oxide by the scraping of the surface over each other. In particular, zirconium alloys are particularly prone to the direct oxidation of the metal by the scraping action.
It is readily contemplated that the continued contamination of the joints between the inner and outer grid straps 20 and 22 and the guide sleeves 36 of a fuel rod grid 16 will eventually lead to the joint's failure. As a result, the fuel rods 18 are subject to intense vibrations due to the high flow of the water, leading to the subsequent fuel rod rupture and to the release of the uranium oxide into the coolant water. Most of this uranium is absorbed by the ion exchangers, but small amounts may also be deposited on core components. The release of the uranium oxide into the water coolant further enhances the corrosion rate not only of the fuel grid 16 but also of the fuel rods 18. The article by Lunde particularly notes that the welding of grid and rod materials such as zirconium alloys in a contaminated welding atmosphere leads to contaminated welds and thus the problems enumerated above. In particular, there is discussed the problem of tungsten welding of Zircaloy and of the adverse effect of oxygen and water in the welding atmosphere. High amounts of oxygen will increase the hardness of the weld.
A further article, entitled "External Corrosion of Cladding in PWRs", by Stehle et al., and appearing in NUCLEAR ENGINEERING AND DESIGN, (1975), particularly describes the effect of corrosion of Zircaloy noting that at temperatures in excess of 500.degree. C. that the presence of oxygen reduces the ductility of this metal. The Stehle et al. article particularly discloses that the main problem of tungsten arc welding is the contamination by impurities in the shielding gas, including fuel particles or tungsten electrode material. In particular, such contamination appears in the form of uranium oxide that appears as a heavy white oxide layer on the fuel rods 18. In particular, the Stehle et al. article suggests that the concentrations of water and oxygen be maintained at below about 20 and 10 ppm, respectively. Though the Lunde and Stehle et al. articles do not deal with the problems of welding large Zircaloy elements and, in particular, fuel rod grids 16 made of Zircaloy, experience has shown that welds produced in a relatively impure atmosphere will produce a weld with an initially low degree of contamination that, when subjected to the harsh atmosphere of a nuclear reactor, will be particularly subject to fretting contamination. Thus, it is particularly critical that any welding of Zircaloy and, in particular, laser welding be conducted in a controlled, pure atmosphere to ensure that weld contamination is minimized and will not deteriorate under the hostile conditions of a nuclear reactor.
U.S. Pat. No. 3,555,239 of Kerth is an early example of a large body of prior art disclosing automated laser welding apparatus in which the position of the work piece, as well as the welding process, is controlled by a digital computer. Kerth shows the control of laser beams while controlling the work piece as it is moved from side to side along an X axis, horizontally forward and backward along a Y axis and vertically up and down along a Z-axis. Typically, pulse driven motors are energized by the digital computer to move the work piece rectilinearly along a selected axis. In addition, the welding is carried out within a controlled atmosphere and, in particular, the pressure and flow of gas into the welding chamber is controlled by the digital computer. Further, a counter is used to count pulses, whereby the number of laser pulses applied to the work piece may likewise be controlled.
U.S. Pat. No. 4,088,890 of Waters discloses a programmable controller for controlling laser emission and, in particular, the control of a high beam shutter whereby the desired quantity of laser emission is directed onto the work piece. This patent also discloses the rectilinear movement of a carriage carrying the work piece along a vertical axis, whereby the work piece is successfully brought to a position, where a laser weld is made. In particular, there is disclosed the effecting of a seam weld, whereby the work piece is rotated while the laser beam is directed at a seam between two pieces to be welded together.
U.S. Pat. No. 3,422,246 of Wetzel discloses a laser cutting machine tool including a servo system for controlling servo drive motors to drive a work piece along X and Y drive axes respectively. A transducer is associated with each of the servo motors to provide feedback signals indicative of the movement of the work piece along its respective axis to thereby ensure accurate work piece position.
Laser machining systems have been adapted for precision work including the cutting of semiconductor assemblies. When lasers are applied to such high precision machining operations, it is important that the relative position between the laser and the work piece, and more particularly between the laser and the work piece holding assembly, be accurately and precisely maintained. Unlike a mechanical tool, which is brought into actual contact with the work piece, a laser is removed from its work piece by a relatively great distance. In such laser machining systems, the work holding means is disposed at a relatively great distance from the laser. In such machines, any relative movement between the work holding means and the laser is amplified by the support structure, causing relatively large movement of the laser beam with respect to the work piece and thus reducing the accuracy with which the laser beam is focused onto the work piece. Relative vibrations between the laser and the work holding means is serious in proportion to the desired small cross section of the laser beam at the work piece. With accuracies measured in thousandths of an inch or less, it is seen that if there is relative vibration, the focused beam will likewise be vibrated, such vibration being amplified by the structural support system by which the laser and the work piece position means are coupled together. For these reasons, vibration has resulted in problems in obtaining the desired accuracy in laser machining, especially where laser beams are focused to very fine points. Such accurate focusing of the laser beam is advantageously done over a relatively long focal length but increases the difficulty of achieving accurate machining due to vibration and shocks. U.S. Pat. No. 3,803,379 of McKay suggests the use of a rigid construction of a laser optical system mounting bed and a work piece holding frame. In particular, the bed is constructed as a hollow box and interconnected by locator pins forced fit through holes in one member and being threaded in the other member. In addition, the laser mounting bed and the work piece holding frame are mounted on vibration isolating pads to support the entire weight of the bed on a rigid floor.
The noted U.S. Pat. No. 3,803,379 also discusses the problem of maintaining the intensity of a laser beam at precise levels. In particular, this patent notes that when a work piece is changed, it is typically necessary to shut down the laser while a new work piece is being installed and thereafter, to start up the laser bringing it back to a desired level of intensity before resuming machining with its laser beam. In particular, the change of the laser beam intensity will effect corresponding changes in the machining effect on the work piece. To overcome this problem, U.S. Pat. No. 3,803,379 suggests that a diverter mechanism be incorporated along the path of the laser beam, whereby the laser beam may be diverted into a heat sink. Thus, while the work piece is being replaced, the diverter mechanism diverts the laser beam into the heat sink, thus allowing the laser to keep firing at a uniform rate without being shut down so that its temperature, once established under equilibrium conditions, will not be altered between machining operations. Further, experience has shown that with heavy laser usage, the intensity of the laser beam will attenuate with time due to aging of the laser itself as well as of the excitation lamps associated therewith. In addition, the laser beam upon striking a work piece typically throws off gaseous material and debris that may coat the work piece or the laser focusing lens, whereby the machining efficiency is attenuated. Thus it is necessary to periodically calibrate the laser system, whereby the energy level of the laser beam as imparted to the work piece may be accurately controlled.
In the initial development of laser machining systems, lasers were employed for individual, low production machining operations. With the development of the art, laser systems were increasingly employed for high production work processing operations as would be controlled automatically by computers. As described above, such high production systems operate efficiently to reposition the work piece, whereby a sequence of welds or other machining operations may be rapidly performed. Under such demands of continuing excitation, laser life becomes a factor in terms of efficient operation and of cost of production. It is contemplated that under high usage where repeated welds are required, as for the production of the above described fuel rod grids, that laser life would be a significant factor to consider. Under heavy usage, the life expectancy of the lamps exciting the pulsed laser would be in the order of several days, and after this life had been expended, it would be necessary to replace at least the lamps, as well as to calibrate the new laser system.
In order to improve laser efficiency and life, the prior art as illustrated by U.S. Pat. Nos. 4,223,201 and 4,223,202 of Peters et al. and U.S. Pat. No. 4,083,629 of Kocher et al., discloses the time sharing of a laser beam emitted from a single laser and alternatively directed along first and second optical paths onto a single work piece. U.S. Pat. No. 4,083,629 describes problems with automated welding systems wherein the work piece requires a plurality of welds to be made; in particular, the work piece may be brought to a first station, whereat a first welder is operated, and then transferred to a second station, whereat a second welder effects a welding operation. Alternatively, two welders could be used at a single station to effect the plurality of welds, thus minimizing the need to transport the work piece from the first to the second stations. However, these methods require that either the work piece be transported or reoriented, thus decreasing the production rate, or that two welders be used thus substantially increasing the capital investment of such apparatus. As an attempt to overcome these problems, U.S. Pat. No. 4,083,629 suggests the use of a bimodal switching means, whereby the laser welder sequentially welds at two distinct weld sites. In particular, there is suggested a motor for rotating a reflected mirror, whereby the beam is alternately directed along a first and then a second focal path to the work piece to effect first and second welds on a single work piece such as an electrical component. It is apparent that there is an automated control of the laser welder to synchronize the firing of the laser with the switching of the laser beams, the wire cutting, and other handling operations. U.S. Pat. No. 4,223,201 describes a somewhat similar laser system adapted for larger work pieces such as would be encountered in ship construction. In particular, U.S. Pat. No. 4,223,201 suggests the use of a rotating mirror to sequentially direct the laser beam along first and second paths, whereby a single laser beam may be time shared. In addition, a suitable automatic controller is employed to control corresponding first and second welding heads that are moved in time relationship with the beam sharing, so as to effect sequentially a series of welds at two different locations on a single work piece. U.S. Pat. No. 4,223,202 suggests the seam welding of two pieces together with the welding taking place on opposite sides of the work pieces at substantially the same point to effect a two sided laser seam weld, while the automated controller effects movement of the welding heads with respect to the work piece.
U.S. Pat. No. 4,078,167 of Banas et al. recognizes the problem of atmospheric contamination of the weld site during laser welding. Laser welding in a vacuum has been attempted, but this patent notes that vacuum welding limits the size and shape of the work piece that can be accommodated as well as increases welding time required to create the vacuum condition. Alternatively, the work piece may be totally immersed in an inert gas, or a trailer shield may provide a flow of known inert gas such as argon over the area of the work piece to be welded. In particular, U.S. Pat. No. 4,078,167 discloses a shield housing for establishing an inert atmosphere about the weld location of the work piece as the work piece is transported beneath the shield housing. An inert gas, typically argon, is passed through a gas passing means having a plurality of openings therethrough for providing a uniform blanket of inert gas which flows over the work piece and through a passage between the shield housing and the work piece into the atmosphere. The flow of inert gas prevents to a degree atmospheric gases including oxygen and water from flowing into the welding zone. It is stated that the flow rate of an inert gas is controlled to shield the weld from reactive gases, but causes turbulence of the melted material which would produce porous and uneven welds.
U.S. Pat. No. 4,078,167 does not mention the particular metal to be welded and does not contemplate the laser welding of Zircaloy as for the above-described fuel rod grid. Zircaloy is known to be highly reactive to oxygen, nitrogen, and water as found in the atmosphere, and welding tests leading to this invention have demonstrated conclusively that inert gas flow around the immediate weld area does not provide adequate shielding for the laser welding of Zircaloy. In accordance with the teachings of this invention, an atmosphere of an inert gas such as argon has been established with a purity in the order of 10 PPM, which degree of purity is not contemplated by U.S. Pat. No. 4,078,167.
The above discussion of the prior art illustrates the significant problems in achieving automated laser welding of a highly reactive material such as Zircaloy, wherein the work piece is sequentially moved under an automated controller to effect a number of precision welds. As enumerated above, it is necessary to move the work piece, e.g., the laser weld grid 16 as described above, along each of its X, Y, and Z axes with respect to the focused laser beam while maintaining an exceptionally high degree of purity of the surrounding atmosphere to avoid contamination of the welded material. In addition, it is desired to achieve a high degree of laser efficiency, even while the work piece is being moved through a sequence of positions in three dimensions with respect to the laser beam. In addition, there are problems of effecting precise welds in parts of small dimensions and, in particular, of maintaining the power level of the impinging laser beam at precise levels for different types of welds, noting the attenuation of laser output as a laser system including the laser rod and excitation lamps is used at high work duty ratios over an extended period of time and the effect of laser welding debris.